1. Field of the Invention
The present invention relates to a thread cutting unit associated with a bed of the sewing machine.
2. Description of the Prior Art
A thread cutting unit of this type is conventionally a knife for cuttting off the stitching thread composed of a needle thread and a bobbin thread at a selected position of the sewn cloth, namely at a position between the throat plate on which the sewn cloth is placed and a hook during which a needle moves to an upper stop position raised from the surface of the sewn cloth after completion of the stitching operation. The sewing machine having such an arrangement is called a thread cutting sewing machine.
In such a conventional thread cutting unit, it is desirable to cut off the needle thread and the bobbin thread by the knife at the position under the throat plate which is as close as possible to the under surface of the sewn cloth after completion of the stitching operation in order to permit the cut end of both threads to remain as long as possible at the side of the sewing machine.
Furthermore, when the needle thread and the bobbin thread are cut off at the portion which is as close as possible to the throat plate, the thread ends of both the needle thread and the bobbin thread remaining on the sewn cloth are so shortened and a later process for removing the thread ends from the sewn cloth is not needed.
However, in a sewing machine a gap between the throat plate and the hook is in general very narrow whereas the known thread cutting units are complex and large. Hence it was difficult to cut off both the needle thread and the bobbin thread at the ideal position close to the sewn cloth.
For example, the thread cutting unit disclosed in the Japanese Patent Laid-Open Publication No. 58-41597 has been designed to solve the problems set forth above by increasing the gap between the hook and the throat plate at the time of thread cutting operation and the thread cutting knife and the like are driven via a link mechanism for converting the linear motion generated by a solenoid to rotational motion. However, since the operation of the thread cutting knife is carried out only by the solenoid independently of the sewing operation, the operating timing thereof is not precise and the lengths of the thread ends of the cut needle thread and the bobbin thread are not uniform but variable.